This invention relates to an objective for a vide disc for a high-density information recording disc, and more particularly to an objective which is used for a video disc using a semiconductor laser as a light source.
A video disc using a semiconductor laser as a light source is well known in the art. In this system, a laser beam from the semiconductor laser is focused on the mirror surface of the disc by an objective, and the laser beam reflected by the mirror surface of the disc is returned through the objective to the semiconductor laser, more specifically to the laser beam generating point, so that the information on the disc is detected according to the intensity of the laser beam thus returned. In this operation, in order that the tracking and the focusing are carried out satisfactorily, the semiconductor laser is moved together with the objective. Accordingly, the movable objective lens must be small in size and light in weight. Furthermore, since it is required for the objective to read the information which is recorded on the disc with high density, the objective lens should have a resolution power of at least one micron, and accordingly the objective should be sufficiently larger in diameter, or it should have a numerical aperture (N.A.) of about 0.5.
This invention has developed an objective system small in size and weight for a video disc, which satisfies specific conditions and has a sufficiently long operating distance or backfocus and in which various aberrations are satisfactorily corrected.